Stator design for a stepping motor is constrained by equations relating the pitch angle, number of poles used, number of phases used, number of rotor teeth used, and other parameters. Stator design is also constrained by practical manufacturing considerations. Adjacent stator teeth with pitch angles less than about 4.degree. are difficult or impossible to construct for stepping motors of a size normally used for semiconductor fabrication. Use of a smaller pitch angle allows greater resolution, if all other factors remain about the same, so that decreasing the stator pitch angle is desirable. With present thin metal stamping technology, the lower limit on the step angle for a four-phase motor of reasonable size is about 0.9.degree.. In principle, a 0.45.degree. step angle would require a rotor diameter of at least 4.4 cm, which is too large for many applications.
Another problem of stepping motor design is to increase the number of stator poles for a fixed number of phases, such as four or eight, for a motor of fixed size. This would allow an improvement in step response. However, conventional approaches again confront manufacturing limitations, and only a modest number of poles can be included in a motor of rotor diameter 4 cm or less.
Kuo et al, in U.S. Pat. No. 3,809,990, disclose use of three coaxial, magnetically independent stator sections or laminations with alternating polarity, the stator teeth of one section being angularly offset from the teeth of each of the other stator sections. The apparatus operates in a stepping mode or in a continuous mode.
In U.S. Pat. No. 3,866,104, Heine discloses a five-phase stepping motor in which a first winding, then a second winding, then a third winding, then a fourth winding, then a fifth winding is short-circuited, one winding at a time, so that the non-energized winding moves from one pole group to another in succession.
Use of two coaxial rotors, angularly offset from one another and separated by a non-magnetic spacer, and eight uniformly spaced stator poles with identical stator and rotor pitch angles, is disclosed by Field in U.S. Pat. No. 4,025,810. Stator teeth at two opposing positions 180.degree. apart are aligned with the adjacent rotor teeth, and stator poles located at the 90.degree. and 270.degree. positions are completely misaligned with the adjacent rotor teeth. The stator teeth at the 45.degree., 135.degree., 225.degree. and 315.degree. positions are intermediate between these orientations with respect to the adjacent rotor teeth. The stator pole windings are alternatingly energized and non-energized. Field, in U.S. Pat. No. 4,255,696, discloses another invention using two coaxial rotor sections with rotor teeth angularly displaced relative to one another.
Manson discloses a stepping motor that uses two coaxial stator sections, positioned back-to-back with a magnetic spacer therebetween and subjected to magnetically independent energization in U.S. Pat. No. 4,355,248. However, the angular rotation, if any, of one stator section relative to the other stator section is unclear.
Use of two identical, coaxial stator sections, positioned back-to-back, is disclosed in U.S. Pat. No. 4,623,809, issued to Westley. Again, it is unclear from the discussion whether the two stator sections are angularly offset relative to one another.
What is needed are stator and rotor designs that allow reduction in step angles to angles much less than 1.degree. and/or allow an increase in the number of stator poles, consistent with currently available manufacturing techniques.